1. Field of the Invention
The present invention relates to a method for manufacturing a langasite single crystal substrate, a langasite single crystal substrate, and a piezoelectric device using the substrate, such as a piezoelectric resonator and a surface acoustic wave (SAW) device.
2. Description of the Related Art
Conventionally, a single crystal substrate, including a piezoelectric substrate, is subjected mainly to wet etching after polishing for the purpose of cleaning the surface. On the other hand, a single crystal substrate material for surface acoustic wave (SAW) use is subjected to roughening the rear surface side, in order to eliminate the effects of reflection of the bulk waves at the rear surface, the waves propagating across the substrate. Thus, the difference of the states of the front and rear surfaces will cause warps. Since these warps exert influences on the cracking in the substrate or on the accuracy of photolithography in forming an electrode, the wet etching treatment is applied as a general measure to alleviate the warps of the substrate.
As an etching solution (etchant) for use in manufacturing a langasite (La3Ga5SiO14) substrate, an acidic aqueous solution comprising HCI, HF and H2O is known (for example, xe2x80x9cEtching LANGASITE and Quartz Crystalsxe2x80x9d, Proceedings of the IEEE International Frequency Control Symposium (1994), pp. 245-250).
This is an aqueous solution obtained by mixing HCI, HF and H2O at a volume ratio of 1:50:150. It is heated to 70xc2x0 C. for etching. It was reported that when polishing was performed using a colloidal silica, the surface state at the time of polishing treatment was maintained after etching for two hours when this etching solution was used, and a good surface state was obtained.
However, as a result of our experiments for confirming the effects of the etching method using a conventional etching solution, it was found that film was formed on the surface of a substrate, the film being supposedly made of LaF3. Although some improvement was observed by increasing the amount of HCI in the etching solution, it was not possible to completely eliminate the LaF3 film. Furthermore, since the etching temperature was as relatively high as 70xc2x0 C., a time-based change in the compositional ratio of the etching solution occurred due to the evaporation of H2O, and, therefore, it was difficult to maintain a similar etching state. Furthermore, because the etchant contains hydrofluoric acid, it was necessary to put more effort to secure safety regarding the apparatuses and the waste water treatment, resulting in more complicate handling.
Accordingly, an object of the present invention is to provide a method for manufacturing a langasite single crystal substrate which furnishes a good surface state, by an etching treatment in a short time, at a relatively low temperature and without forming a film on the substrate surface. Other objects of the present invention are to provide a langasite single crystal substrate obtained by the above-described method and to produce a piezoelectric device using it.
To achieve the above-described objectives, the method for manufacturing a langasite single crystal substrate according to the present invention comprises the step of polishing at least one of the main surfaces of a raw material substrate and the step of wet-etching the above-described main surface of the polished substrate with a solution including H3PO4, HNO3 and CH3COOH.
In the langasite single crystal substrate according to the present invention, at least one of the main surfaces of the single crystal substrate is wet-etched with a solution including H3PO4, HNO3 and CH3COOH.
A piezoelectric device according to the present invention comprises a langasite single crystal substrate, wherein at least one of the main surfaces of the single crystal substrate is wet-etched with a solution including H3PO4, HNO3 and CH3COOH and comprises a transducer formed on the above-described one of the main surfaces of the single crystal substrate.
It is noted that the langasite single crystal substrate recited in the present invention is a piezoelectric single crystal substrate having a Ca3Ga2Ge4O14-type crystal structure.
According to the manufacturing method of the present invention, it is possible to obtain a langasite single crystal substrate having a good surface state by an etching treatment in a short time, at a relatively low temperature and without forming a film on the surface of a substrate. Therefore, such a langasite single crystal substrate can be used to manufacture a piezoelectric device such as a surface acoustic wave device with good properties.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.